This invention relates to a frame saw for sawing of essentially horizontally fed timber by the type of saw having saw blades placed largely perpendicular to the direction of feed of the timber, i.e. without overhang. In such a frame saw a sash in which the said saw blades are clamped is arranged to be imparted a reciprocating upward and downward motion with upper and lower turning points in relation to and controlled by a system of guides which by means of one or several guide connecting rods and via one or several controlled guide links is arranged to be moved before the sash is phase-displaced in the direction of feed of the timber. The guide system and the guide connecting rods are designed with fulcrums in or in relation to said guide links, which are pivotably disposed.
The object of the present invention is to improve in gangsawing the cutting circumstances of the saw blades, or in other words to reduce blade stresses. A reduction of the blade stresses makes it possible to use thinner saw blades, a circumstance which gives smaller kerf losses and thus a higher timber yield. Moreover, it becomes possible to increase the production capacity per machine and unit of time.
In principle, a frame saw consists of a sash which is usually guided by vertical guides, saw blades being fastened in the said sash. The sash is driven up and down in most cases by a connecting rod and crankshaft. The timber is fed through the sash--towards the saw blades--and is then sawn apart by means of a plurality of mutually parallelly disposed saw blades, the numbers of which commonly varies between four and nine, depending on the size of the timber and how it is to be sawn.
Since a frame saw, in terms of function, resembles a reciprocating piston machine, the speed of the saw blades and thus also the cutting effect of the saw blades, will be a sinusoidal function in respect to the cutting period. In prior art conventional frame saw designs, the imperfect machine design in combination with the varying shape of the speed (sinusoidal function) of the saw blades give rise to certain difficulties and disadvantages which will be described in summary below.
The saw blades have their maximum speed in the middle of the stroke (when the crank is horizontal), and when the crank is in its upper and lower turning point respectively the saw blades are stationary. The saw blade speed has a different shape during the cutting period, a circumstance implying that the chip thickness per saw blade tooth varies within wide limits during each cutting period. The cutting period comprises only that part of each crankshaft revolution when the saw blades have downward motion. Normally, the cutting period of the saw blades commences at a crank angle of approx. 10.degree. to 15.degree. after the upper turning point and ends approx. 15.degree. before the lower turning point.
In the beginning and particularly towards the end of the cutting period, the chip thickness per saw blade tooth becomes very large, and in the middle of the stroke, when the saw blades have a maximum cutting speed, it is not possible--paradoxically enough--to take advantage of the maximum cutting effect of the saw blades. Better utilization of the cutting effect of the saw blades in the middle of the stroke can, in conventional frame saws, only take place by increasing the feed rate of the timber. The increase in speed thereby attainable is, however, merely marginal, as every increase in the feed rate leads to a considerable increase in the blade stresses towards the end of the cutting period. At the end of the cutting period--when the saw blade speed is decreasing--from a crank angle of approx. 25.degree. to the lower turning point, the cutting effect of the saw blades is so low that the saw blades chop into the timber and the feed thereof is retarded with the consequence that the saw blades are exposed to very great both horizontal and vertical loads. The horizontal stresses amount to approx. 300 to 600 N per saw blade tooth in deal frames and to approx. 1000 to 3000 N per saw blade tooth in edge frames.
The total load from the workpiece against the saw blades will be approx. 6,000 to 12,000 N in deal frames and approx. 20,000 to 60,000 N in edge frames. The vertical stresses are so great as to cause saw blade teeth to be broken off and the saw blades to tear off. The only possibility of limiting these difficulties and disadvantages in present-day frame saw structures is to design the saw blade teeth with a relatively small clearance angle so that the saw blades do not chop into the timber excessively deeply.
Towards the end of the cutting period--when the saw blades have engaged in the timber--the saw blades break off the lowest part of the saw cut in the workpiece.
The thickness of the broken-off silver may be approx. 5-8 mm and the width equivalent to twice the saw cut width. The thickness of the sliver is measured in the cutting direction of the saw blades themselves and the aforesaid thickness corresponds to a crank angle of approx. 10.degree. to 15.degree. towards the end of the cutting period. It is during this "sliver-forming period" that the retardation of the saw blades by the timber is at its greatest, a circumstance implying that it is during the final phase of the cutting period that the saw blades are exposed to maximum stresses.
It has previously been mentioned that the saw blades perform cutting work only during that part of each crankshaft revolution during which the saw blades have downward motion. It is thus desirable for the saw blades, during their upward motion, to be clear of the bottom of the saw cut. Attempts have been made to solve this problem by inclining the saw blades in the direction of feed (so-called overhang) as then the saw blades will move away from the bottom of the saw cut during their upward motion. Such prior art arrangements are disclosed for example by Swedish Pat. No. 194 103, German Offenlegungsschrift Nos. 1 453 181, 1 528 044, 2 721 841 and through Swiss Pat. No. 391 271.
There is some justification for the overhang design per se but unfortunately with this design, it is not possible to completely avoid so-called back sawing. This commences at the lower turning point and continues until a crank angle of approx. 65.degree.-80.degree. during the upward motion of the saw blades. The reason why back sawing occurs is that the sinusoidal speed of the saw blades does not increase sufficiently quickly in relation to the fed timber.
If the function design of the conventional frame saws is divided according to the position of the crank (the crank angle), the following break-down, starting from the upper turning point, is obtained:
______________________________________ Upper turning Saw blade speed = 0. point Crank angle The saw blades are clear of the saw cut bottom. 0.degree.-15.degree. Crank angle The saw blades commence cutting. Low cutting 15.degree.-25.degree. speed. Less effective cutting work. Large chip thickness. Crank angle During this crank angle, the cutting speed is 25.degree.-150.degree. high. The cutting capacity of the saw blades cannot be fully utilized. Crank angle The cutting speed of the saw blades is de- 150.degree.-165.degree. creasing. Less effective cutting work. Large chip thickness. Crank angle The saw blades stop cutting and retard the 165.degree.-180.degree. timber. The mass forces in the timber and the pulling force from the feeder press the timber towards the saw blades and the tips of the teeth penetrate into the timber without cutting. The saw blades break a sliver from the lower side of the timber. Crank angle Saw blade speed = 0. 180.degree. Crank angle The saw blades have upward motion. The timber is 180.degree.-250.degree. pressed against the saw blades. Back sawing. Crank angle The saw blades have upward motion. The saw 250.degree.-360.degree. blades run clear of the bottom of the cut in the timber. ______________________________________
The following general remarks are applicable to the conventional saw frame:
1. The cutting speed of the saw blades follows a sinusoidal function and during a crank angle of approx. 25.degree. after the upper turning point and approx. 30.degree. before the lower turning point, the cutting effect of the saw blades is good and the blade stresses relatively small.
2. Around the turning points of the saw blades, the cutting effect thereof is poor and the blade stresses are very great.
3. After the lower turning point of the saw blades--when the saw blades have upward motion--back-sawing occurs, a negative phenomenon which damages both saw blades and timber.
Such prior art arrangements are disclosed by for example German Pat. No. 881 258 and German Offenlegungsschrift Nos. 2 721 842 and 2 638 964. The closest prior art devices are disclosed by the Applicant's own Swedish Pat. No. 215 830 and U.S. Pat. No. 3,322,170.
In principle, an object of the present invention is for the cutting period of the saw blades to be located at that portion of each crankshaft revolution during which the saw blades have sufficient cutting effect and during the remaining portion of the crankshaft revolution, the saw blades must be clear of the bottom of the cut.
Eliminated by this means are the large unfavourable loads which affect the saw blades and this in turn enables sawing to be performed with saw blades having substantially smaller thicknesses than the saw blades used in present-day conventional frame saws.
The present invention also enables sawing to be carried out with virtually constant chip thickness per tooth tip, a circumstance which is of the utmost importance with regard to both the surface fineness of the machined timber and for elimination of forces unfavourable to the cutting process.
The aforesaid difficulties and disadvantages of the conventional frame saws give rise to great stresses in the saw blades and this results in the necessity of the saw blades having large thickness in order not to achieve a wavy saw cut with resultant poor dimensional accuracy of the sawn timber.
Saw blades with large thicknesses, moreover, necessitate large clamping forces in the sash, a circumstance which gives a heavy machine structure with large reciprocating masses and a low speed, which gives low cutting capacity per unit of time.
Saw blades with large thicknesses give large cutting losses and poor production economy.
By application of the present invention, it becomes possible to eliminate the difficulties and disadvantages inherent in conventional frame saw designs.